The present invention generally relates to organic light-emitting devices, and in particular to organic light-emitting devices having reduced ambient-light reflection.
An organic light-emitting diode, also referred to as an organic electroluminescent (EL) device is constructed in a normal configuration on a light-transmissive substrate through which the light emitted by the device is viewed, and the device structure comprises, in sequence, the light-transmissive substrate, a light-transmissive conductive hole-injecting electrode, an organic hole-transporting layer, an organic light-emitting layer, an organic electron-transporting layer and an electron-injecting electrode consisting of a metal having a work function less than about 4.0 eV. This highly reflective metal electrode helps to improve brightness of emission in that the electron-injecting electrode provides a surface from which internally generated light from the emission layer or from a light-emitting junction is reflected and directed toward the light-transmissive substrate. However, such a metallic electron-injecting electrode also reflects ambient-light entering the device structure through the light-transmissive substrate and the light-transmissive hole-injecting electrode, thereby degrading the visually perceived contrast of the emitted light, as viewed by an observer. In numerous practical applications, it is quite important that an organic light-emitting device can be easily viewed under ambient-lighting conditions ranging from total darkness to full sunlight so that a sufficient reduction is required in reflection of ambient-light from the mirror-like surface of the metal electron-injecting electrode.
In the construction of some inorganic light-emitting devices, one approach to enhanced sunlight readability and reduction of glare has been to incorporate in such an inorganic device an absorbing and dielectric layer between the inorganic phosphor layer and the counter electrode layer. The thickness of the dielectric layer is optimized to create destructive optical interference of the ambient-light, thereby reducing ambient-light reflection. This approach has produced inorganic light-emitting displays having 14% total reflectance, as disclosed in the Journal of Military and Aerospace Electronics, Volume 9, No. 6, June, 1998, which describes features of such a device created by Luxell Technologies, Inc.
Another well known approach to reducing glare attributed to ambient-lighting is to use polarizers, particularly circular polarizers, which may be bonded to an outside surface of the light-transmissive substrate. However, the use of polarizers adds significant cost and a polarizer bonded to a substrate is not a part of the integral layer structure of a light-emitting device. Furthermore, polarizers may reduce the emitted light intensity by 60 percent.
Returning to the aforementioned inorganic light-emitting device, such device has a metal-insulator-semiconductor-insulator-metal-thin-film configuration. Therefore, a dielectric and absorbing material can be used between the top metal electrode and the phosphor emission layer for the reduction in reflection of ambient-light from the top electrode.
In contrast to inorganic light-emitting devices, organic EL devices require electron injection into an organic electron-transporting layer, as well as hole injection into an organic hole-transporting layer. Electron-hole recombination at or near a junction between these organic layers results in light emission when the hole-injecting electrode (also referred to as the anode) is biased at a sufficient positive electrical potential with respect to the electron-injecting electrode (also referred to as the cathode).
To provide effective electron injection from the cathode into the organic electron-transporting layer, the cathode must be formed of a metal or of a combination of metals selected to provide a work function of less than 4.0 eV. If the cathode is formed of a metal having a work function greater than 4.0 eV, electron injection from the cathode to the electron transporting layer will be significantly reduced.
Due to the construction and function of organic EL devices, an ambient-light reflection-reducing layer disposed between a reflective cathode and the electron-transporting layer to provide optical absorbance, has to be electrically conductive, provide a work function of less than 4.0 eV, and be formable by deposition techniques which are compatible with organic EL device fabrication so as to minimize deleterious effects such as, for example, radiation damage or undesirable chemical or physical interactions between the reflection-reducing layer and the organic layer or layers.
Thus, the requirements and specifications imposed on an ambient-light reflection-reducing layer useful in organic light-emitting devices are substantially different from and more stringent than, the requirements for such reflection-reducing layer or structure for an inorganic light-emitting device.
It is an object of the present invention to provide an organic light-emitting device having reduced ambient-light reflection from a cathode.
It is another object of the present invention to provide an organic light-emitting device having reduced ambient-light reflection from a cathode, in which a reflection-reducing layer is formed between the cathode and an organic electron-transporting layer, and of an electrically conductive material having a work function less than 4.0 eV.
It is a further object of the present invention to provide an organic light-emitting device having reduced ambient-light reflection from a cathode, in which a reflection-reducing layer is formed between a bi-layer interfacial structure and the cathode, and of a material selected to be an n-type semiconductor having a work function greater than 4.0 eV. In one aspect, these and other objects of the invention are achieved in an organic light-emitting device having reduced ambient-light reflection from a cathode, comprising:
a) a light-transmissive substrate;
b) a light-transmissive anode disposed over the substrate;
c) an organic hole-transporting layer disposed over the anode;
d) an organic electron-transporting layer disposed over the hole-transporting layer, an interface between the electron-transporting layer and the hole-transporting layer forming a junction capable of emitting light when the device is operative;
e) a reflection-reducing layer disposed over the electron-transporting layer, the reflection-reducing layer formed of an electrically conductive material having a work function less than 4.0 eV and capable of providing electron injection into the electron-transporting layer, the reflection-reducing layer substantially reducing reflection of ambient-light entering the device through the substrate and the anode; and
f) a cathode disposed over the reflection-reducing layer, the cathode formed of a light-reflecting metal material.
In another aspect, these and other objects of the invention are achieved in an organic light-emitting device having reduced ambient-light reflection from a cathode, comprising:
a) a light-transmissive substrate;
b) a light-transmissive anode disposed over the substrate;
c) an organic hole-transporting layer disposed over the anode;
d) an organic electron-transporting layer disposed over the hole-transporting layer, an interface between the electron-transporting layer and the hole-transporting layer forming a junction capable of emitting light when the device is operative;
e) a bi-layer interfacial structure disposed over the electron-transporting layer, the bi-layer interfacial structure capable of providing electron injection into the electron-transporting layer;
f) a reflection-reducing layer disposed over the bi-layer interfacial structure, the reflection-reducing layer formed of an n-type semiconductor material having a work function greater than 4.0 eV, the reflection-reducing layer substantially reducing reflection of ambient-light entering the device through the substrate and the anode; and
g) a cathode disposed over the reflection-reducing layer, the cathode formed of a light-reflecting metal material.